1. Field of the Invention
The present invention relates generally to the bands Adaptive Modulation and Coding (AMC) method in an Orthogonal Frequency Division Multiple Access (OFDMA) communication system, and in particular, to an adaptive subchannel and bit allocation method using feedback information in an OFDMA communication system.
2. Description of the Related Art
Research into the 4th generation (4G) mobile communication system, which is the next generation communication system, is being conducted to provide users with services having various Qualities-of-Service (QoSs) at a data rate of about 100 Mbps. Compared with the 4G communication system, the current 3rd generation (3G) mobile communication system supports a data rate of about 384 Kbps in an outdoor channel environment providing relatively poor channel conditions and supports a data rate of 2 Mbps at most even in an indoor channel environment providing relatively good channel conditions.
A wireless Local Area Network (LAN) system and a wireless Metropolitan Area Network (MAN) system generally support a data rate of 20 to 50 Mbps. Currently, a study of the 4G mobile communication system is being actively performed to develop a new communication system capable of supporting mobility and QoS in the wireless LAN system and the wireless MAN system, both of which guarantee a relatively high data rate, thereby to support a high-speed service that the 4G mobile communication system aims to provide. An Orthogonal Frequency Division Multiplexing (OFDM) scheme has emerged as a promising scheme useful for high-speed data transmission on wired/wireless channels in the 4G communication system. The OFDM scheme, a typical scheme for transmitting data using multiple carriers, is a type of Multi-Carrier Modulation (MCM) scheme for parallel-converting a serial input symbol stream and modulating each of the symbols with the multiple orthogonal subcarriers, or subcarrier channels, before transmission.
A multiple access scheme based on the OFDM scheme is known as the OFDMA scheme. The OFDMA scheme allows multiple users (i.e. mobile stations) to share the subcarriers of one OFDM symbol. Communication systems using the OFDMA scheme include Institute of Electrical and Electronics Engineers (IEEE) 802.16a, 802.16d and 802.16e communication systems. The IEEE 802.16d communication system is a communication system employing the OFDMA scheme to support a broadband transmission network to physical channels for the wireless MAN system. The IEEE 802.16d communication system is a typical example of a Broadband Wireless Access (BWA) communication system using a TDD (Time Division Duplex)-OFDMA scheme. The IEEE 802.16d communication system, as it applies the OFDMA scheme to the wireless MAN system, can support high-speed data transmission by transmitting physical channel signals using a plurality of subcarriers.
With reference to FIG. 1, a description will now be made of a frame structure for the OFDMA communication system.
Referring to FIG. 1, a full subcarrier band is divided into a plurality of bands, each band including a plurality of bins or tiles. Each bin or tile has a plurality of subcarriers. A bin is the set of 9 consecutive subcarriers within an OFDM symbol, having 1 pilot tone and 8 data tones, and a tile is composed of the set of 3 consecutive subcarriers in 6 consecutive OFDM symbols, having 2 pilot tones and 16 data tones.
In a frame, the first three OFDM symbols are used for a ranging channel, a Hybrid Automatic Repeat reQuest (HARQ) channel, and a Channel Quality Information (CQI) channel, while the other OFDM symbols are allocated to the bands AMC channel, a diversity channel, and a safety channel.
The leading band AMC channel is allocated in units of band comprised of 6 bins, and the following diversity channel is allocated in units of subchannel comprised of three tiles spread over the full subcarrier band. The band AMC channel, occupying more OFDM symbols than the diversity channel, can be used to transmit/receive a large amount of data at a high rate by applying a modulation scheme with a high coding efficiency when reception quality is good.
A safety channel occupies one bin across all OFDM symbols. The safety channel is allocated all symbols of one bin. A mobile station is allocated a safety channel with a frequency band allocable in a base station in a safety channel unused in a neighboring cell, i.e., a frequency band that remains unallocated.
To allocate an appropriate channel to a mobile station according to its state, a base station must have knowledge of the state of the mobile station. Therefore, if the data quality of the mobile station is decreasing, the base station transmits a CQI request message to the mobile station, for channel reallocation.
In the above-described OFDMA communication system, CQI messages for all bands are periodically fed back to the base station. This feedback can be expressed as “4 bits/band×32 bands×200 Hz×32 users=819.2 Kbps” in the case of FIG. 1.
The feedback information is very large in size in the conventional technology, decreasing transmission efficiency. Therefore, there is a need for developing a CQI feedback and AMC scheme that exhibits the same performance as that achieved in the case where CQIs of all bands are used with a less amount of feedback information.